The present invention relates to a grid used in a storage battery.
Lead-acid storage batteries for electric vehicles and automobiles are required to have excellent high rate discharge characteristics. In producing a grid for a lead-acid storage battery, there are a casting method, which produces a grid by pouring molten lead into a mold, an expanding method, which produces a grid by the steps of applying slitting to a sheet of lead or lead alloy prepared in advance by a rolling method or the like and then expanding the slitted sheet, and some other methods. The casting method is not efficient since the operation is not continuous. In addition, depending on kinds of lead alloy, it may be difficult for the casting method to produce a thin grid that is required to improve high rate discharge characteristics, resulting in a drawback associated with this casting method.
The expanding method shows excellent productivity because of the continuous operation that is made possible with this method and also has a benefit of enabling the production of a thin electrode just by adjusting the thickness of belt-shaped material. Therefore, the expanding method has been recently used widely. A case where a grid is produced by the expanding method will be explained below. FIG. 6(a) is a plan view and a side view of a prior art grid and FIG. 6(b) is a partially enlarged view of the grid of FIG. 6(a). As shown in FIG. 6(a) and FIG. 6(b), the width of each respective connecting section 5 of strand 4 is about twice the incision width of strand 4 with the prior art grid. In other words, the connecting section 5 is larger than the grid rib 4 in thickness. As clearly seen in the side view of FIG. 6(a), a grid 20 shows a configuration, in which short straight-line segments are overlapping one another, and the overlaps come to coincide with the connecting sections 5. Further, the connecting sections 5 are slanting against the electrode surface, thereby causing the edges of the connecting sections 5 to project from the electrode surface.
In order to achieve enhanced durability with a lead-acid storage battery, it is desirable to apply pressure to an electrode group of the lead-acid storage battery. By applying pressure to the electrode group, an active material can be prevented from peeling and falling off the grid during a cycle lapse that is accompanied by expansion and contraction of the active material. Therefore, efforts have been made, on the other hand, to develop a material and a geometry for a battery container that can withstand the high pressure applied to the electrode group.
FIG. 7 is a cross-sectional view of an electrode, which uses such a prior art grid 20 as described in the above and provided with an active material. As indicated in FIG. 7, the elasticity of the connecting sections 5 of strand 4 of the grid is small when compared with the strand 4 of the grid, where an active material 6 is filled in. Therefore, as the pressure applied to the electrode group increases, a separator 7 is crushed at places where the separator 7 is in contact with pressure applied points 8 of the electrode, where the application of pressure is extensively strong, thereby blocking the supply of electrolyte to the foregoing pressure applied points 8 and/or creating an accumulation of gas at the pressure applied points 8. As a result, localized degradation of the active material 6 is caused to occur. Moreover, degradation of the crushed separator 7 multiplies and a short circuit at the pressure applied point 8 develops, thereby ending up with shortening of the battery""s life.
A storage battery of the present invention comprises an electrode and an electrolyte material, in which the electrode has a grid and an active material is provided on the grid; and the grid has a plurality of strand and a plurality of connecting sections, where the strand cross one another; and in case where a pressure is applied to the foregoing grid in the thickness direction thereof, the grid is made to have such a configuration as to have the above pressure applied to each respective connecting section in the direction perpendicular to the afore-mentioned thickness direction of the grid.
What is particularly desirable is:
the plurality of strand as mentioned in the above have a first grid rib and a second grid rib that are arranged so as to have each of the afore-mentioned connecting sections located between the first grid rib and the second grid rib;
each of the foregoing first strand and second strand has a first projecting member and a second projecting member, respectively, with the above first projecting members and second projecting members projected opposite to one another, respectively, in the aforementioned thickness direction of the grid; and
when a pressure is applied to the foregoing first projecting member in the direction vertical to the surface of the grid, the above pressure is applied to the connecting section in the direction perpendicular to the foregoing thickness direction of the grid.
Further, what is particularly desirable is:
the foregoing grid is an expanded grid;
each of the foregoing first strand is curved so as to have the foregoing projecting member located at the top of the curvature;
and each of the foregoing second strand is curved so as to have the foregoing projecting member located at the top of the curvature.
Accordingly, when an external pressure is applied to a grid in the thickness direction thereof, the pressure is propagated in the direction perpendicular to the thickness direction of the grid, thereby preventing the pressure from propagating in other directions than the direction perpendicular to the thickness direction of the grid. Therefore, when a pressure is applied to an electrode in a storage battery using an electrode group, which includes electrodes provided with an active material in grids and separators, connecting sections of the grid are prevented from applying the pressure to the separators via the active material, thereby enabling a uniform supply of an electrolyte material to electrodes. Furthermore, gas accumulation is also prevented from occurring. As a result, localized degradation of the active material is prevented and battery""s life is extended.
By the use of a grid with strand curving upward and downward of the thickness direction, the curved ribs perform a function of springs and distribute the pressure applied to the grid in both the direction of pressure application and the vertical direction. Accordingly, the pressure applied to the active material and grid is made uniform, resulting in the prevention of localized degradation of the active material and separators and the extended battery""s life.